This invention relates to a method of producing a glass substrate for a perpendicular magnetic recording disk.
Data processors for recording and reproducing data such as characters, images and sounds are coming to be installed not only in computers but also in apparatus such as televisions, cameras and telephones. Such data processors are now required to have improved processing capabilities (with increased recording capacities) and accuracy in reproduction and to be smaller in size. Data are magnetically recorded on a magnetic recording medium and reproduced therefrom by means of a magnetic head of the data processor.
As disclosed in http://www.trl.ibm.com/projects/perpen/ (“Perpendicular Magnetic Recording”, IBM Tokyo Research Laboratory) and http://spin.pe.titech.ac.jp/hp/research/nfts2/(“Production of Co—Cr High-Density Perpendicular Magnetic Recording Medium”, Nakagawa Group, Department of Electronic Physical Engineering, Tokyo Engineering University), perpendicular magnetic recording disks are now under consideration as a magnetic recording medium. Such disks are produced by sequentially forming a magnetic layer and a protective layer on the surface of a disk-shaped glass substrate by using a thin film technology such as sputtering. The magnetic layer comprises an assembly of columnar crystalline elements having a segregated structure by composition separation of a magnetic layer material deposited on the surface of a high-temperature glass substrate, and each crystalline element is comprised of a ferromagnetic columnar center part extending in a direction perpendicular to the surface of the glass substrate and a non-magnetic surrounding part formed around this center part. These columnar crystalline elements form the recording bits that are magnetizable in the direction perpendicular to the surface of the glass substrate.
Because a magnetic layer is thus formed with columnar crystalline elements extending perpendicularly to the surface of the glass substrate, the surface of a perpendicular magnetic recording disk is particularly required to be flat and smooth such that the average surface roughness should be 2Å or less.
The increase in the capacity for data recording and the accuracy in reproduction both depend largely on the distance of separation between the surface of the magnetic disk and the magnetic head. Since data are recorded by outputting a magnetic signal from the magnetic head to form small magnets on the magnetic layer and reproduced by reading the magnetic signals from these small magnets by means of the magnetic head, an increased distance of separation between the surface of the magnetic disk and the magnetic head means that the magnetic signals outputted from the magnetic head is dispersed more such that the quantity of recording per unit area (the recording density or recording capacity) is reduced. Thus, in order to increase the capacity of data recording and to improve the accuracy of reproduction, the distance of separation between the surface of the magnetic disk and the magnetic head must be made smaller. Moreover, the magnetic disk can be made smaller if the recording quantity per unit area is increased. For this reason, the distance of separation between the surface of the magnetic disk and the magnetic head is now required to be 15 nm or less.
Magnetic heads are either of the floating type or the contacting type, as explained, say, in http://www.jst.go.jp/pr/report/report22/(“Success in Development of Contacting Type Thin Film Magnetic Heads for Hard Disk”, Report No. 22, Kagaku Gijutsu Shingo Jigyodan). Magnetic heads of the floating type are provided with a slider on the side opposite the magnetic disk so as to stabilize the head at a floating distance (the distance to the magnetic disk) of 15 nm or less. If the unevenness in the height of the magnetic disk surface is large, the slider of the magnetic head may contact or collide with the uneven surface to damage the magnetic disk and it will not be possible to stably maintain a floating distance of 15 nm or less. Magnetic heads of the contacting type are adapted to contact the surface of the magnetic disk through an elastic pad but if the magnetic disk has an uneven or rough surface, the magnetic head may be caused to oscillate and damaged.
The glass substrate for a perpendicular magnetic recording disk is therefore required to have a high level of smoothness (with average surface roughness of 2.0Å or less) and a high level of flatness (with the surface height variations of 1Å or less with wavelengths in the range of 0.05 mm-0.5 mm in both radial and circumferential directions).
In general, glass substrates are polished with free abrading particles by using a lapping plate or a tape. According to Japanese Patent Publication Tokkai 9-314458, slurry with abrading particles with average particle diameter of 10 nm-1 μm of a material such as artificial or natural diamond, cerium oxide and zirconium oxide dispersed at a rate of 0.5 weight %-20 weight % with respect to the whole of the slurry is used for the polishing. With prior art slurry of this kind, however, the surface roughness of the polished surface of a glass substrate exceeds 5Å, and it is not possible to obtain a surface with average roughness equal to or less than 2Å, as desired.